Superluminescent light emitting diodes (SLEDs) are diodes that, when biased in the forward direction, become optically active and generate amplified spontaneous emission over a wide range of wavelengths.
SLEDs (sometimes also called Superluminescent diodes, SLDs) are attractive for applications in which a higher intensity than the one emitted by conventional LEDs is required, but where an even distribution of the emitted wavelength over a broad spectral range is desired. In a SLED for delivering a large incoherent light output from a first end facet, it is thus important to suppress laser oscillation.
In contrast to laser diodes, therefore, there is not sufficient feedback to obtain lasing action (“lasing” here is used to describe the function principle of a laser, i.e. to generate, by a feedback, stimulated emission in a gain medium pumped to provide population inversion and placed in a cavity providing the feedback, resulting in coherent radiation). This is usually achieved by the joint action of a tilted waveguide in which the generated radiation is guided and anti-reflection coated end facets. A tilted waveguide in this context is a waveguide which is not perpendicular to a plane defined by end facets of the device.
In U.S. patent application Ser. No. 10/763,508, which is incorporated herein by reference, a new method of suppressing laser oscillation has been described. According to this method, electrodes in an absorber region are kept at zero voltage so that absorption is enhanced.
Among the properties which are usually desired for SLEDs are a large spectral width and a high temperature stability. For this reason, quantum dot superluminescent diodes are promising. In such diodes, the gain medium is formed by a high quantity of quantum dots, which have usually been produced by self-assembly, such as by epitaxial growth of a quantum dot layer in the Volmer-Weber growth mode or in the Stranski-Krastanov growth mode. A large spectral width is achieved by a naturally occurring inhomogeneous size distribution leading to different electronic structures between the different quantum dots. High temperature stability occurs because of the non-continuous density of states, where the energy difference between neighboring states exceeds usual values of kT (k being Boltzmann's constant and T being the absolute temperature).
Although the inhomogeneous size distribution of the quantum dots brings about a relatively large spectral width naturally, it would be advantageous to even further increase the spectral width. For this purpose, it has been proposed to deliberately increase the dot size inhomogeneity distribution (Z.-Z. Sun et al., Optical and Quantum Electronics 31, p. 1235–1246 (1999)). However, the exact control of the quantum dot size dispersions is neither trivial nor easily reproducible. A different approach proposed was to use multiple layers with InAs quantum dots with different amounts of deposited InAs material in the quantum dots (Z. Y. Zhang et al., IEEE Photonics Technology Letters 16, p. 27–29 (2004)). Since the amount of InAs also affects the density and radiative efficiency of the quantum dots (QDs), this last approach is difficult to implement, too.
Other electroluminescent elements in which a broadband emission spectrum is desired include Semiconductor Optical Amplifiers in which spontaneous emission is used for amplifying incoming radiation (of potentially a broad bandwidth) and external cavity semiconductor lasers, in which a large emission spectrum is desired in order to be able to tune the laser output in a large range.